There have been known electromagnetic wave shielding filters that are mounted on the front of a variety of displays, such as PDPs (plasma display panels) and CRT (cathode ray tube) displays, in order to shield electromagnetic waves that the displays generate. The electromagnetic wave shielding filters to be used for this purpose are required to have not only electromagnetic wave shielding properties but also light transmission properties. However, this requirement cannot fully be satisfied by an electromagnetic wave shielding filter produced by forming an ITO (indium tin oxide) film on the entire surface of a transparent substrate (see Patent Documents 1 and 2, for example). Accordingly, there has been proposed an electromagnetic wave shielding filter comprising metal foil, such as copper foil, laminated to a transparent resin film substrate by an adhesive, etched into a mesh (see Patent Document 3, for example).
Patent Document 1: Japanese Laid-Open Patent Publication No. 278800/1989,
Patent Document 2: Japanese Laid-Open Patent Publication No. 323101/1993, and
Patent Document 3: Japanese Laid-Open Patent Publication No. 210988/2001.
Further, front filters or the like to be mounted on the front of displays are sometimes required to have not only the function of shielding electromagnetic waves but also other functions such as the function of shielding unwanted light that displays radiate (for example, when mounted on PDPs, light with wavelengths around 590 nm produced by neon emission) to compensate for the color tone of displayed images for enhancing color reproducibility, the function of preventing unwanted reflection of extraneous light, and the function of suppressing unwanted radiation of infrared rays from displays to avoid malfunction of infrared ray equipment. Furthermore, the front filters are also required to be light in weight and small in thickness. Therefore, when an electromagnetic wave shielding filter has only the function of shielding electromagnetic waves, it is often made into a composite filter, for practical use, by integrally laminating it to a filter having another function, such as an antireflection filter, a coloring filter, or an infrared absorption filter (see Patent Documents 2 and 3, for example).
In the case where a coloring filter containing a coloring agent is laminated to an electromagnetic wave shielding filter for the purpose of shielding unwanted visible light and infrared rays that a display itself radiates, or unwanted light such as extraneous light (reflection), no problem has occurred as long as a coloring filter with a transparent substrate is simply laminated to a mesh layer in an electromagnetic wave shielding filter by an adhesive such as a pressure-sensitive adhesive, with the transparent substrate of the coloring filter facing to the mesh layer. FIG. 7(A) shows an example of a conventional electromagnetic wave shielding filter 20 having the above-described structure, that is, an electromagnetic wave shielding composite filter 20 in which a functional layer 8, such as an optical filter, composed of a transparent substrate 8B and a functioning layer 8A formed on the transparent substrate 8B is laminated to a mesh-laminated sheet 21 composed of a transparent substrate 1 and a mesh layer 3 laminated to the transparent substrate 1 with an adhesive layer 22 containing no coloring agent, with the transparent substrate 8B of the functional layer 8 facing to the mesh layer 3 of the mesh-laminated sheet 21. The functioning layer 8A is, for example, a transparent resin layer in which a near infrared absorber has been incorporated so that the layer has the function of absorbing near infrared rays, and, in this case, the functional layer 8 serves as a near infrared absorption filter.
In this structure, the functioning layer 8A is isolated from the metal mesh layer 3 by the transparent substrate 8B and the adhesive layer 22. Therefore, even if the functioning layer 8A has the possibility that it might undergo a change in color when it interacts with the mesh layer, no such interaction actually occurs due to the transparent substrate 8B and the adhesive layer 22.
However, in order to make the electromagnetic wave shielding composite filter lighter in weight and smaller in thickness, and to decrease the number of the constituent layers of the composite filter (which leads to decrease in cost and haze and to improvement in light transmission properties), if the adhesive that is applied when the electromagnetic wave shielding filter and the functional layer such as the above-described coloring filter are laminated is colored with a coloring agent so that the adhesive layer can also serve as a coloring filter, another problem has newly occurred. Namely, it was found that an adhesive colored with a coloring agent undergoes a change in color (discolors or fades) with time.
FIG. 7(B) shows an example of a conventional electromagnetic wave shielding filter 20 having the above-described structure, that is, an electromagnetic wave shielding composite filter 20 in which a functional layer 8, such as an optical filter, composed of a transparent substrate 8B and a functioning layer 8A formed on the transparent substrate 8B is laminated to a mesh-laminated sheet 21 composed of a transparent substrate 1 and a mesh layer 3 laminated to the transparent substrate 1 with a transparent colored resin layer 4 serving also as an adhesive layer, made from a transparent resin to which a coloring agent serving as a neon light absorber, for example, has been added so that the resin layer has the function of cutting neon light, with the transparent substrate 8B of the functional layer 8 facing to the mesh layer 3 of the mesh-laminated sheet 21. The functioning layer 8A is, for example, a layer of a transparent resin in which a near infrared absorber has been incorporated so that the layer has the function of absorbing near infrared rays, and the functional layer 8 thus functions as a near infrared absorption filter. In this case, it was found that the neon-light-cutting properties deteriorate with time, or that unfavorable coloring (discoloration) occurs.
It was also found that the coloring agent tends to cause color deterioration especially when a transparent adhesive layer 9 is present between the transparent substrate 1 and the mesh layer 3, or when the mesh layer 3 contains a blackening layer 7, as shown in FIG. 7(C).